Problem: $ { \left[\begin{array}{rr}{1} & {-2} \\ {-1} & {-1}\end{array}\right]+\left[\begin{array}{rr}{-1} & {4} \\ {3} & {4}\end{array}\right] = {?} } $
Explanation: To find the sum of two matrices, add the corresponding items between each matrix. ${\left[\begin{array}{rr}{1}+{-1} & {-2}+{4} \\ {-1}+{3} & {-1}+{4}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{0} & {2} \\ {2} & {3}\end{array}\right]}$